The investigation of physical factors affecting success of bone marrow transplantation experiments was continued and included both radiobiological effects of the exposure and subtle changes in physical factors that alter survival of irradiated inbred mice with and without marrow grafts. Basic concepts of radiation biology were demonstrated to be invalid and physical factors considered to have insignificant effects were demonstrated to be critical for the reproducibility of data. Subtle changes in the quality of X-ray resulted in significant differences in results. Exposure-rate effects and exposure-rate in combination with dose reduction by absorption resulted in altered patterns of repair of lethal and sublethal cellular damage and were a function of the direction of the exposure. The heterogeneous nature of the hematopoietic system which consists of circulating cells, cells encased in skeletal bone and in soft tissue of the spleen, thymus, and lymph nodes and the potential for a single normal or neoplastic cell to repopulate the system required greater emphasis on physical conditions of the radiation than has been considered necessary in the past. More precise methods of determining the physical parameters as they affect the experimental data should be devised. In addition, the practice of converting calibrated exposures in air to absorbed tissue doses by using inaccurate and inappropriate coefficients of absorption should be abandoned in all cases where the exposure to a specific tissue or organ can not be measured directly. There is a need for a radical change in the way physical factors are viewed when dealing with biological systems.